Long-term variations in the lower stratosphere, troposphere, and upper ocean during recent winter seasons, and their interrelationship are documented. The two leading modes of the interannual-interdecadal variability are extracted using a singular value decomposition (SVD) analysis of the unfiltered Northern Hemisphere (NH) 500hPa height and quasi-global sea surface temperature (SST). We also conducted a SVD analysis of the winter 500hPa height and the lagged SST to get insights into their causal relationship. Then, we investigated the relationship between the tropospheric and stratospheric circulation using a SVD analysis of the unfiltered NH 500 and 50hPa height fields.
Highly significant correlation is found both (i) between the first SVD mode of the 500hPa height and SST (SVD_SST-1) and the second SVD mode of the 500 and 50hPa height (SVD_Z50-2) and (ii) between the second SVD mode of the 500hPa height and SST (SVD_SST-2) and the first SVD mode of the 500 and 50hPa height (SVD_Z50-1). SVD_SST-1 and SVD_Z50-2 capture a striking ENSO (El Nino/Southern Oscillation) signal. They also capture stepwise interdecadal rising and falling of the tropical Pacific SST and 500hPa height over the North Pacific, respectively, both of which occurred in the mid 1970's. The lagged SVD suggests that the enhanced westerlies over the North Pacific forced the cooling of the underlying SST. The associated height anomalies in the lower stratosphere are characterized by the zonal wave number 2 components.
The other mode of the global variability represented by SVD_SST-2 and SVD_Z50-1 may be considered as the internal variability in the atmosphere. This mode represents the variability in the strength of the winter stratospheric polar vortex, and it is characterized by the tropospheric North Atlantic Oscillation (NAO) and distinct height anomalies over East Asia. This mode also influences the mid-latitude SST by changing the low-level wind. The associated SST anomalies appear as a meridional tripole pattern in the North Atlantic that develops from winter to spring. The time coefficients of this mode exhibit a predominant decadal signal superimposed on a linear trend, with an abrupt sign reversal at the end of the 1980's. The presence of this mode may suggest a possible link between the atmospheric internal variability and anomalies in the Earth's surface conditions or somewhere else in the climate system which acts as long-term "memory".